In recent years, a surface acoustic wave (hereinafter, SAW) device is widely utilized as apart for a terminal for mobile communication, a vehicle-mounted equipment, or the like, and it is strongly demanded to be downsized, have a high Q value, and be excellent in frequency stability.
As a SAW device realizing these demands, there is a SAW device using an ST cut quartz substrate. The ST cut quartz substrate is a cut name of a quartz plate having a plane (XZ′ plane) obtained by rotating an XZ plane from a crystal Z-axis in a counterclockwise direction by an angle of 42.75° utilizing a crystal X-axis as a rotation axis, and it utilizes a SAW (hereinafter, called ST cut quartz SAW) that is a (P+SV) wave propagated in a crystal X direction, called “Rayleigh wave”. An application of the ST cut quartz SAW device widely lies in a SAW resonator used as an oscillator, a filter for IF disposed between an RF stage and an IC in a mobile communication terminal, or the like.
The reason why the ST cut quartz SAW device allows realization of a downsized device with a high Q value includes a point such that reflection of SAW can be utilized efficiently. An ST cut quartz SAW resonator shown in FIG. 13 will be explained as one example. The ST cut quartz SAW resonator has a structure that interdigital electrodes (hereinafter, called IDT) 102 having pluralities of electrode fingers mutually inserted are disposed on an ST cut quartz substrate 101 and grating reflectors 103a and 103b for reflecting SAW are disposed on both sides of the IDTs 102. Since the ST cut quartz SAW is a wave propagated along a surface of a piezoelectric substrate, it is efficiently reflected by the grating reflectors 103a and 103b and SAW energy can be sufficiently confined in the IDTs 102, so that a downsized device with a high Q value can be obtained.
As an important factor in use of the SAW device, there is a frequency-temperature characteristic. In the ST cut quartz SAW, it is generally known that the primary temperature coefficient of the frequency-temperature characteristic of the SAW is zero, a characteristic thereof is expressed by a quadratic curve, and the SAW is excellent in frequency stability because a frequency fluctuation amount is significantly reduced by making an adjustment so that a turnover temperature is positioned at the center in a usage temperature range.
However, in the ST cut quartz SAW device, while the primary temperature coefficient is zero, the secondary temperature coefficient is relatively large such as −0.034 (ppm/° C.2). Accordingly, when the usage temperature range is expanded, the frequency fluctuation amount becomes extremely large.
As an approach for solving the problem, there is a SAW device disclosed in Meirion Lewis, “Surface Skimming Bulk Wave, SSBW”, IEEE Ultrasonics Symp. Proc., pp. 744 to 752 (1977), and Japanese Examined Patent Publication No. 62-016050. As shown in FIG. 14, the feature of the SAW device is characterized in that a cut angle θ of a rotation Y cut quartz substrate is set near a position rotated from a crystal Z-axis in a counterclockwise direction by an angle of −50°, and a propagation direction of SAW is set to a perpendicular direction (a Z′-axis direction) to a crystal X-axis. Incidentally, when the cut angle is expressed by Euler angle, (0°, θ+90°, 90°)=(0°, 40°, 90°) is obtained. The SAW device is characterized in that an SH wave propagated just below a surface of a piezoelectric substrate is exited by an IDT and oscillation energy is confined just below an electrode. A frequency-temperature characteristic of the SAW device forms a cubic curve, and since a frequency fluctuation amount in a usage temperature range is significantly reduced, an excellent frequency-temperature characteristic is obtained.
However, since the SH wave is a wave that fundamentally submerges in a substrate to advance, a reflecting efficiency of a SAW obtained by a grating reflector is poorer than that in the ST cut quartz SAW propagated along the piezoelectric substrate surface. Accordingly, there is a problem that it is difficult to realize a downsized SAW device with high Q. Since the prior Publication includes disclosure about application as a delay line which does not utilize reflection of a SAW but it does not propose any means for utilizing reflection of a SAW, it is said to be difficult to put the SAW device in a practical use.
In order to solve the problem, Japanese Examined Patent Publication No. 01-034411 discloses a so-called multi-pair IDT type SAW resonator in which, as shown in FIG. 15, a cut angle θ of a rotation Y cut quartz substrate is set in the vicinity of −50° and multiple pairs, such as 800±200 pairs, of IDTs 112 is formed on a piezoelectric substrate 111 where a propagation direction of a SAW is set in a perpendicular direction (the Z′-axis direction) to a crystal X-axis, so that high Q is achieved by confining SAW energy by only reflection of the IDTs 112 themselves without utilizing a grating reflector.
However, the multi-pair IDT type SAW resonator cannot obtain efficient energy confining effect as compared with a SAW resonator including a grating reflector. Since the number of IDT pairs required to obtain a high Q value is increased considerably to 800±200 pairs, there is a problem that a device size becomes larger than an ST cut quartz SAW resonator, so that a recent demand for size reduction cannot be satisfied.
In the SAW resonator disclosed in Japanese Examined Patent Publication No. 01-034411, when a wavelength of a SAW excited by the IDT is represented as λ, a Q value can be increased by setting an electrode film thickness to 2% λ or more, preferably 4% λ or less. However, when a resonant frequency is 200 MHz, the Q value reaches saturation near 4% λ, but the Q value obtained at that time becomes only about 20000 and only the Q value approximately equal to that in the ST cut quartz SAW resonator can be obtained in a comparison with the ST cut quartz SAW resonator. As its reason, it is thought that since, when the film thickness is in a range of 2% λ or more to 4% λ or less, a SAW are not concentrated sufficiently on a piezoelectric substrate surface, reflection cannot be utilized efficiently.
[Patent Document 1]Japanese Examined Patent PublicationNo. 62-016050[Patent Document 2]Japanese Examined Patent PublicationNo. 01-034411[Nonpatent Document 1]Meirion Lewis, “Surface Skimming BulkWave, SSBW”, IEEE Ultrasonics Symp.Proc., pp. 744 to 752 (1977)